Best to read the NHS website which gives the details you seek. The answers are there.
I did read the instructions/advice that come with the app and I don't remember that it answered my question. However, I have now taken your advice and stopped being so lazy and looked it up for myself. The answer appears to be only when you have been in proximity of someone who has
tested positive. Which makes sense. However it stops short of explaining distance and duration.
I might add that the 'NHS app' term is most confusing as there are not 1 but 2.
1 The NHS covid19 app we are discussing here.
2 The NHS app. This is new and I was directed to it in order to complete an electronic referral form for my wife called 'eConsult'. This took most of the morning as the security is massive. email conformation as well as passport photo check. It does give you access to your records. But the again, new app which referred me to the NHS app 'AIRMID' also gave me that access. Furthermore, we ere referred to the airmid app by its processor 'SYMTOMS ON LINE' which we have been using for a few years. My friend, who is chair of the patient advisory committee says the surgery want to keep the system on line' open, and they don't consider it to be extinct.
So 3 ways of doing almost the same thing. Is this progress, I really don't think so.
To add. Once registered with the NHS app we could complete eConsult form. This is similar to the 111 questions which drive you wild as they never seem to let you get to the point.
On the plus side, someone from the surgery contacted us within 20 mins.
Simplicity should always be the key to such things. Also, the writers should always have their systems checked for usability by the end users.
John